The present invention relates generally to a method of and system for correcting scatter in a computed tomography (CT) scanner, and more particularly to a method of and system for determining the x-ray scatter present in the scanner chamber prior to obtaining scans of the target object, in order to compensate for scatter caused by static elements in the scanner chamber.
Elastic scattering results from the interaction of photons of an x-ray beam with atoms of a scanned object. The x-ray photons cause electrons of the scanned object to vibrate while still bound to their orbits around the nuclei. The electrons re-radiate the x-ray energy in all directions. The amount of scattering depends on the effective atomic number of the impinged atom, as it will increase for atoms having a higher atomic number. Since elastic scattering is a resonant phenomenon the electrons remain bound to the atoms, and the photons are not absorbed by the scanned object. Compton scattering, which is more significant, is due to the direct exchange of energy between the x-ray photon and an electron with which it collides. Part of the photon energy is absorbed by the electron and converted into kinetic energy. The photon is then scattered at a lower energy level. While most of the photons are attenuated within the scanned object, a small portion of photons are not absorbed, resulting in an increase in scatter radiation as part of the primary x-ray beam.
Scattering causes artifacts in images reconstructed from the x-rays. It adversely affects image contrast and generates streaks from high-density objects. The increase in counts due to scattering reduces the measured densities, resulting in reduced contrast of scanned objects in the reconstructed images. Scattering also causes cupping and blurring similar to the beam hardening artifact in large bulk objects.
An exact solution to the problems caused by scatter cannot be determined due to the randomness of the scattering process. Some prior art systems utilize anti-scatter plates which are disposed between the detectors of a detector array and which act to reduce the amount of scatter that reaches each detector, so that the detector receives mostly x-rays that travel to the detector in a direction substantially perpendicular to the detector. However, anti-scatter plates are extremely expensive and add structural complexity to the scanners. Another prior art approach to reducing scatter in an x-ray scanner is to estimate the amount of scatter in a scanner system using either constant scatter values or values that are obtained with Monte-Carlo simulations. These systems may or may not utilize anti-scatter plates. However, these systems are not able to account for scatter caused by the scanner tunnel and conveyor, which are within the field of view of the x-ray beam.
The present invention utilizes scatter detectors to measure the amount of scatter within the scanner before an object to be scanned is placed therein and scanned. This scatter data is used to determine the amount of scatter caused by the object being scanned, so that the scatter associated with the object can be compensated for.
According to a first embodiment of the invention, a system for correcting for scatter in a computed tomography scanner includes a tunnel having a platform disposed therein for receiving an object to be scanned, an x-ray source for directing x-rays at the object to be scanned, a detector array including a plurality of primary detectors for receiving the x-rays and at least one secondary detector for receiving portions of the x-ray beam scattered within the tunnel. The system further includes processing means for reducing the effects of scatter in images of the object reconstructed from the x-rays detected by the primary detectors, the processing means performing the steps of:
A. determining an amount of scatter detected by the at least one secondary detector resulting from the tunnel and the platform prior to the placement of the object on the platform;
B. determining an amount of scatter detected by the at least one secondary detector resulting from the tunnel, the platform and the object after the object has been placed on the platform;
C. estimating an amount of scatter caused only by the presence of the object on the platform within the tunnel; and
D. correcting the reconstructed images of the object based on the amount of scatter estimated in Step C.
According to another embodiment of the present invention, a method of correcting for scatter in a computed tomography scanner having a platform for receiving an object to be scanned, the platform being disposed within a tunnel, a primary detector array for detecting x-rays provided by and x-ray source and a secondary detector array for detecting scattered x-rays from the x-ray source is disclosed. The method includes:
A. determining an amount of scatter detected by the secondary detector array resulting from the tunnel and the platform prior to the placement of the object on the platform;
B. determining an amount of scatter detected by the secondary detector array resulting from the tunnel, the platform and the object after the object has been placed on the platform;
C. estimating an amount of scatter caused only by the presence of the object on the platform within the tunnel; and
D. correcting reconstructed images of the object based on the amount of scatter estimated in Step C.